1998 Ford Explorer with automatic transmission. While driving at a steady speed the transmission will down shift, up shift and shift erratic, it seems to do this at all speeds. Very noticeable driving at 45-50 mph, the transmission will shift back and forth between 3rd and 4th gears while driving at a steady speed. The transmission is not slipping. After a couple of days of driving with this problem the check engine light went on, code P0705 (Transmission Range Sensor Circuit malfunction (PRNDL Input)). I replaced the Transmission Range Sensor/Neutral safety switch, still have the same problems. Anyone have any ideas. Thanks

Sounds more to me like an issue with one or more of the transmission or vehicle speed sensors. If you have access to a professional scan tool that can monitor the transmission computer datastream, you can watch the various speed sensors to see if one or more are reading erratically.

-Rod